The present invention relates to an adaptive antenna control method and adaptive antenna transmission/reception characteristic control method, which can be used to, e.g., improve the frequency use efficiency in a radio communication system having a plurality of base stations by suppressing interference from a neighboring base station.
In a radio communication system that forms a planar service area, such as a mobile communication system, radio zones formed by a number of base stations are combined to construct a wide service area. Radio zones formed at separate positions simultaneously use the same frequency as radio signals. With this method, the frequency use efficiency can be improved.
Forming hexagonal zones is most effective to minimize, in each radio zone, interference from the remaining radio zones.
For example, as indicated by reference (Okumura and Shinji, “Fundamentals of Mobile Communications”, p. 195), when a service area is constructed by hexagonal zones, the number K of frequencies required by this radio communication system is given byK=(1/3)×(D/R)2 
D: the distance between base stations of cells (radio zones) which use the same frequency
R: the radius of a cell
When each cell has a regular hexagonal shape, (D/R>3) must hold. Hence, the number K of frequencies is at least three.
For the above reason, to provide a communication service in a wide service area using a conventional typical radio communication system, at least three radio frequencies must be used.
When an adaptive antenna is employed, interference from another radio zone that uses the same frequency can be suppressed.
For example, a typical adaptive antenna as shown in reference (R. A. Monzingo and T. W. Miller, “Introduction to Adaptive Arrays”, John Wiley & Sons, Inc. 1980) has an arrangement shown in FIG. 9.
Referring to FIG. 9, this adaptive antenna comprises N antenna elements 901(1) to 901(N), weighting circuits 902(1) to 902(N) and 912(1) to 912(N), weight control unit 903, reference signal generation unit 904, divider/combiner 905, and distributor 913.
The weighting circuits 902(1) to 902(N) and divider/combiner 905 are used for reception. The weighting circuits 912(1) to 912(N) and distributor 913 are used for transmission. Each weighting circuit 902 weights the signal from a corresponding antenna element 901 with a complex number. The weight control unit 903 controls the value of the weight to be supplied to each weighting circuit 902 or 912. The divider/combiner 905 generates a signal by combining the signals of N systems, which are weighted by the respective weighting circuits 902. The distributor 913 distributes a signal to be transmitted to systems equal in number to the antenna elements 901.
When signals received by the antenna elements 901(1) to 901(N) are represented by x(1) to x(N), the values of weights in the weighting circuits 902(1) to 902(N) are represented by w(1) to w(N), and a desired signal component is represented by d, a weight WOPT for minimizing the error between the desired signal component d and the reception signal obtained at the output of the divider/combiner 905 is given by
                              W          opt                =                              R            xx                          -              1                                ⁢                      r            xd                                              (        13        )                                          R          xx                =                  E          ⁢                      ⌊                                          X                *                            ⁢                              X                T                                      ⌋                                              (        14        )                                          r          xd                =                  (                                                                                                                x                      ⁡                                              (                        1                        )                                                              ·                                          d                      *                                                        _                                                                                                                                                x                      ⁡                                              (                        2                        )                                                              ·                                          d                      *                                                        _                                                                                    ⋮                                                                                                                                x                      ⁡                                              (                        N                        )                                                              ·                                          d                      *                                                        _                                                              )                                    (        15        )                                          X          =                      (                                                                                x                    ⁡                                          (                      1                      )                                                                                                                                        x                    ⁡                                          (                      2                      )                                                                                                                    ⋮                                                                                                  x                    ⁡                                          (                      N                      )                                                                                            )                          ⁢                                  ⁢                              W            opt                    =                      (                                                                                                      w                      opt                                        ⁡                                          (                      1                      )                                                                                                                                                              w                      opt                                        ⁡                                          (                      2                      )                                                                                                                    ⋮                                                                                                                        w                      opt                                        ⁡                                          (                      N                      )                                                                                            )                                              (        16        )            
where
suffix *: conjugate transposition
suffix T: transposition
E[·]: expected value
X: input signal vector
x(i): reception signal of ith antenna element
d: desired signal
wopt(i): weight for ith antenna element
When the directional pattern of the antenna is controlled by generating such a weight, a null is formed in the directional pattern with respect to the direction of an interference station. Hence, the influence of the interference wave from the interference station can be suppressed. A “null” means that the radiation field or reception field strength becomes 0.
By installing an adaptive antenna in a base station, even when, e.g., communication is executed using the same radio frequency in adjacent radio zones, the influence of an interference wave from a neighboring radio zone can be suppressed.
However, assume that a base station uses an adaptive antenna, and another base station (interference station) that uses the same frequency as that of the n station (base station) is present in the direction of a target terminal station viewed from the base station. In this case, if the directional pattern of the antenna is controlled to suppress the influence of the interference wave from the interference station, the signal from the target terminal station is also suppressed, and the transmission quality inevitably degrades.
In a radio communication system, limited frequency resources must be effectively used. However, in a radio communication system which provides a radio communication service in a wide range using a plurality of base stations, as described above, since interference from a neighboring zone to a given base station and interference from the given base station to the neighboring zone are present, zones adjacent to each other cannot use the same frequency.
When an adaptive antenna is used, the interference wave from a neighboring zone can be suppressed, and therefore, the same radio frequency can be used in adjacent radio zones. However, no sufficient interference reduction capability can be obtained only with the control of a conventional adaptive antenna. Especially, when a target terminal station is present in the direction of the zone of the neighboring base station, the interference unavoidably increases.